Abstract
Batan Bay and Tinago Lake are shallow embayments connected to each
other, located on the north of Panay Island, central Philippines (11.53°
– 11.67°N, 122.38° – 122.52°E). Although they had been originally
surrounded by dense mangrove forest till the middle of the last century,
mangroves have been mostly cleared and converted into fish and shrimp
ponds. Recently, shelves and rafts for cultivating oysters and green
mussels have become widespread in the shallow areas of the embayments
(see Figure as an example). Replantation of mangroves is also ongoing in
limited areas of Batan Bay. We are conducting researches there focusing
on ecosystem services of mangroves and seagrass meadows, especially in
relation to carbon sequestration and aquaculture production. In this
presentation, we report preliminary survey results on environmental
conditions that may influence growth and survival of cultivated
bivalves, such as freshwater inputs and potential food resources. The
survey was conducted in both dry season (February 2019) and rainy season
(November 2019). Although the salinity gradient across the bay due to
freshwater input was evident in both seasons, the oxygen isotope ratio
of seawater indicated that evaporation overwhelmed in inner bay sites in
the dry season. Concentrations of chlorophyll and suspended particulate
organic matter (POM), i.e. potential food source for bivalves, were high
in the inner bay area. Carbon stable isotope ratio (δ13C) of dissolved
organic carbon (DIC) and POM showed spatial gradient from the bay mouth
(high) to inner sites (low), indicating the influence of riverine DIC
and POM inputs. However, the δ13C of oysters (adductor muscle) was
consistently higher than POM and showed no clear spatial gradient. The
δ13C of oysters was relatively higher for individuals collected from
inside or edge of seagrass meadows than those collected in open areas.
These results suggest that oysters assimilate only a specific fraction
of POM relatively enriched in 13C (i.e. marine-origin POM) and that
seagrass meadows support growth of oysters by providing additional food
source (e.g. attached microalgae that are abundant on seagrass blades).